In dye thermal transfer printing, heat is applied to selected pixel areas of a dyesheet to cause dye to transfer from the heated areas to form printed pixels on an adjacent dye receiver sheet and thereby form a printed image.
Laser diodes are often chosen as the heat source, as they are inexpensive, reliable and compact. In typical printing apparatus, the output beam from a single laser diode is intensity modulated as it is scanned across the dyesheet, and each scan prints a line of pixels to the receiver sheet. The beam may be modulated merely on or off, so that a pixel is either printed or not, or the beam intensity may be varied over a range of values to vary the amount of dye transferred from a pixel area and provide pixels of varying tones to allow for continuous tone printing.
Laser diodes, however, typically emit light from an elongate stripe area, and so the beam quality is asymmetric, the output beam having a high divergence in the axis of the stripe's width and a low divergence in the axis of the stripe's length. A problem with laser diodes then is that, although the beam may be focused quite readily to a relatively high resolution in its high divergence axis, it is more difficult to do this in the low divergence axis, and so high overall resolution is difficult to achieve.
A further problem of the above apparatus is that the print speed is low, because each pixel must be individually printed in turn. Also, the beam-scanning optical assembly is complex and increases the costs and size of the apparatus, whilst reducing reliability.
One way of increasing print speed, and of simplifying the scanning assembly, is to arrange a plurality of laser diodes into an array, and to intensity modulate each laser diode individually while scanning the output beams together across the dyesheet. This then allows a plurality of pixel lines to be printed simultaneously. The use of a plurality of laser diodes, however, itself, adds to the expense and complexity of the apparatus.